You list sites where I can find free scripts; should I look into purchasable scripts as well?
If a purchasable script offers the functionality that you'll need, and the amount of money you spend on the script is worth the time you save by buying it, you should definitely purchase commercial software. When you're working with Web applications, you really want to pay attention to the amount of tweaking you can do to commercial software. Generally, applications have to fit within the overall design of your site, and you may want functionality that's not provided by the commercial program. It helps to be able to make changes to the software on your own.
Search engines, content management systems, and message board software are all examples of Web applications that you can purchase to augment your site.
Can I use a debugger to debug CGI programs?
You can if you can run them from the command line and test them there. There's no way to run a debugger on a program being accessed over the Web, but all CGI programs should work fine at the command line as long as you pass them data in the appropriate manner. If you're accustomed to debugging software using an interactive debugger, by all means use your scripts from the command line and debug them that way.